Electronic circuit including a current mirror circuit

ABSTRACT

This disclosed electric circuit substantially eliminates error output which is caused by unbalanced characteristics of a pair of transistors in a current mirror circuit. An input signal is amplified by a differential amplifier. A current mirror circuit is coupled between said differential amplifier and a power supply. Switching means is coupled to said current mirror circuit for cyclically switching input and output points of said mirror circuit.

BACKGROUND OF THE INVENTION

This invention relates to an electronic circuit and more particularly, to a phase comparison circuit and other electronic circuits including a current mirror circuit.

A phase comparison circuit, for example, can utilize an electronic circuit including a current mirror circuit. The circuit can function as a pilot signal detecting circuit in a PLL-MPX (phase locked type-multiplex) decoder IC which demodulates a stereophonic signal. Such a pilot signal detecting circuit is shown in FIG. 1. As shown, a stereophonic signal, which comprises main and sub-audio signals and a pilot signal, is supplied across the base electrodes of transistors Q₁, Q₂. Transistors Q₁, Q₂ constitute a differential amplifier 11. After the stereophonic signal is amplified by amplifier 11, it is supplied from the collector electrodes of Q₁, Q₂ to each common base electrodes of transistors Q₃, Q₄ and transistors Q₅, Q₆. Transistors Q₃ to Q₆ constitute a double balanced differential amplifier 12. A switching signal from a switching signal source 13, which has the same frequency as the pilot signal (e.g., 19KHz) is supplied across the base electrodes of transistors Q₃, Q₆ and transistors Q₄, Q₅. Amplifier 12 multiplies the stereophonic signal by the switching signal and produces an output signal from the collector electrodes of transistors Q₄, Q₅ to an output terminal 15. An AC component in the output signal is supplied to power supply line L₁₁ through a condenser C₁₁, connected between the terminal 15 and line L₁₁. Consequently, a DC level corresponding only to the pilot signal is provided from terminal 15 to a stereo indicative lamp (not shown).

Voltage fluctuation in the power supply are reduced by a current mirror circuit 14, positioned between amplifier 12 and L₁₁, which comprises a pair of transistors Q₇ and Q₈. Although it is desired for transistors Q₇, Q₈ to have identical characteristics so the current can be balanced, this is generally not possible due to the mass production of transistors. In general, therefore, an error current flows from mirror circuit 14 to output terminal 15. If the current gain of circuit 14 is 1+ε, the error current flowing into terminal 15 is (ε/2)I_(o) ; where ε is the error factor due to the unbalance of transistors Q₇, Q₈, and I_(o) is a current value produced by current source I₁₁. Source I₁₁ is connected to the common emitter electrode of transistors Q₁, Q₂. In general, ε equals (approximately) ±0.1; as a result, 5% of the current value I_(o) is the error output current supplied to terminal 15.

This error output current, however, is not negligible. In the stereophonic signal, the modulation factor of the pilot signal is 10% and that of each audio signal is 90%. Accordingly, the level of each audio signal is nine times the level of the pilot signal. Furthermore, the current value I_(o) need be large to secure the dynamic range of the stereophonic signal. If I_(o) is 600 μA, the variation of the pilot signal becomes approximately ±10 μA provided that the upper and lower dynamic ranges of the stereophonic signal is three times the level of the audio signal. If I_(o) is 600 μA, then the error signal current becomes approximately ±30 μA. Consequently, the output including a large error component, in comparison with the pilot signal, will be supplied from terminal 15 to the stereo indicative lamp even when a stereophonic program is not be received. Accordingly, this high level error signal should not be detected.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an electronic circuit which substantially eliminates the error output current produced by an unbalance of the mirror circuit.

Another object of the present invention is to provide an electronic circuit which can be formed in a integrated semiconductor circuit.

According to the present invention, an electronic circuit comprises: a signal input and output terminals; a differential amplifier having at least one input terminal and a pair of output terminals, the input terminal coupled to said signal input terminal; a first transistor having its collector electrode coupled to one of said output terminals of said differential amplifier; a second transistor having its collector electrode coupled to the other output terminal of said differential amplifier, its base electrode coupled to the base electrode of said first transistor; a first switching means having a movable contact, a first stationary contact, and a second stationary contact, its movable contact coupled to the base electrode of said first transistor, its first stationary contact coupled to the collector electrode of said first transistor, and its second stationary contact coupled to the collector electrode of said second transistor; second switching means having a movable contact, first stationary contact and a second stationary contact, its movable contact coupled to said signal output terminal, its first stationary contact coupled to the collector electrode of said first transistor, and its second stationary contact coupled to the collector electrode of said second transistor; and, a control means for cyclically controlling the positioning of said movable contacts of said first and second switching means between said first and second stationary contacts.

The objects and advantages of the present invention will become apparent to persons skilled in the art from a study of the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit diagram of a conventional phase comparison circuit.

FIG. 2 is a schematic circuit diagram explaining the operation and construction of an electronic circuit according to the present invention, for example, shown in FIGS. 3-4.

FIG. 3 is a schematic circuit diagram of a phase comparison circuit according to the present invention.

FIG. 4 is a schematic circuit diagram of another embodiment of a phase comparison circuit according to the present invention.

FIG. 5 is a schematic circuit diagram explaining the operation and constitution of a modification of an electronic circuit according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will be described in detail with reference to the accompanying drawings. Throughout the drawings, like reference numerals will be used to designate like or equivalent portions, for simplicity of explanation.

The fundamental constitution of an electronic circuit according to the present invention is shown in FIG. 2. An input signal is applied across the base electrodes of transistors Q₂₀ and Q₂₁ which form a differential amplifier 20. Their emitter electrodes are grounded through a common current source I₂₁.

An amplified signal is supplied from the collector electrode of each transistor Q₂₀, Q₂₁ to a current mirror circuit 21 comprising transistors Q₂₂ and Q₂₃. The collector electrode of transistor Q₂₂ is coupled to the collector electrode of transistor Q₂₀, while its emitter electrode is coupled to a power supply line 24. The collector electrode of transistor Q₂₃ is coupled to the collector electrode of transistor Q₂₁, while its base electrode is coupled to the base electrode of transistor Q₂₃, and its emitter electrode is coupled to line 24. A first switching means 22 has a movable contact S₁ coupled to base electrode of the transistor Q₂₂, its stationary contact S₂ coupled to the collector electrode of transistor Q₂₂, and its stationary contact S₃ coupled to the collector electrode of transistor Q₂₃. A second switching means 23 has a movable contact S₄ coupled to a signal output terminal 26, its stationary contact S₅ coupled to the collector electrode of transistor Q₂₂, and its stationary contact S₆ coupled to the collector electrode of transistor Q₂₃. A capacitor C₂₁ is connected between terminal 26 and line 24 to average the output supplied by contact S₄. The movement of contacts S₁, S₄ are cyclically controlled by a control means 25 as follows: movable contact S₄ contacts stationary contact S₆ at the time movable contact S₁ contacts stationary contact S₂. During the time, however, movable contact S₄ contacts stationary contact S₅, movable contact S₁ contacts stationary contact S₃. As a result, the error current flowing into output terminal 26 is reduced from (ε/2) I_(o), as in the prior art circuit, to ##EQU1## as will be developed in detail below.

The error output current supplied to output terminal 26 is as follows: It is assumed that the error factor of circuit 21, due to the unbalance of transistors Q₂₂ and Q₂₃, is ε. Control means 25 is a count-down circuit for producing a symmetric square wave output having a one-half duty cycle. When contact S₁ contacts contact S₂, and contact S₄ contacts contact S₆, the current gain of the circuit 21 is 1+ε. When, alternately, contact S₁ contacts contact S₃, and contact S₄ contacts with contact S₅, the current gain of circuit 21 becomes ##EQU2## Consequently, the error output current, which is averaged by condenser C₂₁ and supplied to the terminal 26, is ##EQU3## where I_(o), is a value of current supplied from current source I₂₁.

When ε is 0.1 the error output current is approximately 0.00227 I_(o), namely, 0.23% of the current I_(o). Now, if the current I_(o) is 600 μA, the error output current becomes approximately 1.38 μA. Accordingly, when compared with the error output current of the prior art circuit (i.e., 30 μA), the error current is substantially reduced.

This invention, for example, may be applied to a pilot signal detecting circuit in a PLL-MPX decoder IC which demodulates a stereophonic signal. Such a pilot signal detecting circuit is shown in FIG. 3. As shown, a stereophonic signal, comprising a main and sub-audio signals and a pilot signal (19 KHz), is applied across the base electrodes of transistors Q₂₀, Q₂₁ which form a differential amplifier 20. After the stereophonic signal is amplified by the amplifier 20, it is supplied from the collector electrodes of transistors Q₂₀, Q₂₁ to the respective collector electrode of transistors Q₂₂ and Q₂₃. Transistors Q₂₂, Q₂₃ constitute a current mirror circuit 21. The emitter electrode of transistor Q₂₂ is connected to a power supply line 24, while its base electrode is connected to the base electrode of transistor Q₂₃. The emitter electrode of transistor Q₂₃ is connected to line 24. The collector electrodes of transistors Q₂₄, Q₂₅ are connected to the base electrode of transistor Q₂₂. The emitter electrode of transistor Q₂₄ is connected to the collector electrode of transistor Q₂₂, while its base electrode is connected to the gate electrode of a field-effect transistor FET₂₀. The source electrode of transistor FET₂₀ is connected to the collector electrode of transistor Q₂₃, while its drain electrode is connected to an output terminal 26 and to line 24 through a condenser C₂₁. The emitter electrode of transistor Q₂₅ is connected to the collector electrode of transistor Q₂₃, while its base electrode is connected to the gate electrode of a field-effect transistor FET₂₁. The source electrode of FET₂₁ is connected to the collector electrode of transistor Q₂₂, while its drain electrode is connected to terminal 26. Transistors Q₂₄, Q₂₅, FET₂₀ and FET₂₁ function as switching means 22, 23 (FIG. 2).

One of a pair of output terminals of a 19KHz signal source 25 (e.g., symmetrical square wave) is connected to the base electrode of transistor Q₂₄. The other terminal of source 25 is connected to the base electrode of transistor Q₂₅. Source 25 functions as control means 25 (FIG. 2). As a result of the positive periodic output of source 25, transistor Q₂₄, FET₂₀ conduct while transistors Q₂₅, FET₂₁ are cut off. During the negative periodic output of source 25, transistors Q₂₄, FET₂₀ are cut off while transistors Q₂₅, FET₂₁ conduct. A DC signal, have very little error component and corresponding only to the pilot signal, is provided from terminal 26 to a stereo indicative lamp (not shown). Except for connection pin 27 for line 24, this circuit has only one outside connection pin 28 (i.e., for condenser C₂₁); consequently, it may be easily fabricated into a semiconductor integrated circuit.

Another embodiment of the pilot signal detecting circuit according to the present invention is shown in FIG. 4. For simplicity of explanation, portions similar to the circuit of FIG. 3 have been omitted. The base electrode of a transistor Q₃₀ is connected to one output terminal of 19KHz signal source 25. The base electrode of a transistor Q₃₁ is connected to the other terminal of source 25. The emitter electrodes of transistors Q₃₀, Q₃₁ are grounded through a common current source I₃₁. The emitter electrodes of transistors Q₃₂, Q₃₃ are connected to the collector electrode of transistor Q₃₀, while the base electrode of transistor Q₃₂ is connected to the collector electrode of transistor Q₂₂. The collector electrode of transistor Q₃₂ is connected to power supply line 24. The collector electrode of transistor Q₃₃ is connected to the collector electrode of a transistor Q₃₆ and the base electrode of a transistor Q₃₇. The base electrode of transistor Q₃₆ is connected to the base electrode of transistor Q₂₂, while its emitter electrode is connected to line 24. The emitter electrode of transistor Q₃₇ is connected to the base electrode of transistor Q₂₂, while its collector electrode is grounded. The emitter electrodes of transistors Q₃₄, Q₃₅ are connected to the collector electrodes of transistor Q₃₁. The base electrode of transistor Q₃₅ is connected to the collector electrode of transistor Q₂₃, while its collector electrode is connected to line 24. The base electrode of transistor Q₃₄ is connected to the base electrode of transistor Q₃₃ and a negative electrode of a reference voltage source V₃₀. A positive electrode of source V₃₀ is connected to line 24. The collector electrode of transistor Q₃₄ is connected to the collector electrode of transistor Q₃₃. Transistors Q₃₀ to Q₃₇ function as switching means 22 (FIG. 2). Rather than utilizing a switching circuit for means 23 as shown in FIG. 2, FIG. 4 employs serially connected resistors R₃₀, R₃₁, connected between the collector electrodes of transistors Q₂₂, Q₂₃, to perform a similar function. In this case, the output signal is divided by resistors R₃₀ and R₃₁ to reduce the error signal.

As a result of the output of source 25, transistors Q₃₀, Q₃₁ conduct alternatively. During the positive cycle of source 25 when transistor Q₃₀ conducts and transistor Q₃₁ is cut off, transistors Q₃₂, Q₃₃ conduct and transistors Q₃₄, Q₃₅ are cut off. During the negative cycle of source 25 when transistor Q₃₀ is cut off and transistor Q₃₁ conducts, transistors Q₃₂, Q₃₃ are cut off and transistors Q₃₄, Q₃₅ conduct.

A modification of the electronic circuit according to the present invention is shown in FIG. 5. For example, this circuit can be used as an operational amplifier. As shown, a small offset input voltage is applied across the base electrodes of transistors Q₂₀, Q₂₁ which constitute a differential amplifier 20. The emitter electrodes of transistors R₂₀, R₂₁ are grounded through a common current source I₂₁. The collector electrode of transistor Q₂₀ is connected to a switching means 50. Switching means 50 has a movable contact S₅₁ connected to the collector electrode of a transistor Q₂₀, its stationary contact S₅₂ connected to the collector electrode of a transistor Q₂₂, and its stationary contact S₅₃ connected to the collector electrode of a transistor Q₂₃. The collector electrode of transistor Q₂₁ is connected to a switching means 51 and an output terminal 26. Switching means 51 has a movable contact S₅₄ connected to the collector electrode of the transistor Q₂₁, its stationary contact S₅₆ connected to the collector electrode of transistor Q₂₃ and its stationary contact S₅₅ connected to the collector electrode of transistor Q₂₂. As shown in the previous embodiments, transistors Q₂₂, Q₂₃ constitute a current mirror circuit 21. The emitter electrode of transistor Q₂₂ is connected to a power supply line 24, while its base electrode is connected to the base electrode of transistor Q₂₃. The emitter electrode of transistor Q₂₃ is connected to line 24. A switching means 22 has its movable contact S₁ connected to the base electrode of transistor Q₂₂, its stationary contact S₂ connected to the collector electrode of transistor Q₂₂, and its stationary contact S₃ connected to the collector electrode of transistor Q₂₃. The movements of contacts S₁, S₅₁ and S₅₄ are cyclically controlled by a switching signal from a control means 25, which has a higher frequency than the input signal. During the time movable contact S₁ contacts stationary contact S₂, movable contact S₅₁ contacts stationary contact S₅₂ and movable S₅₄ contacts stationary contact S₅₆. During the time, however, movable contact S₁ contacts stationary contact S₃, movable contact S₅₁ contacts stationary contact S₅₃, and movable contact S₅₄ contacts stationary contact S₅₅.

It should be readily apparent to a person skilled in the art that this invention can be employed in various types of electronic circuits including, for example, a phase comparator circuit in a FM quadrature detecting circuit, a AM synchronous detecting circuit and a phase-locked loop circuit. 

I claim:
 1. An electronic circuit having signal input and output terminals, said electronic circuit comprising:a differential amplifier having at least one input terminal and a pair of output terminals, the input terminal being coupled to said signal input terminal; a first transistor having base, emitter and collector electrodes, the collector electrode being coupled to one of said differential amplifier output terminals; a second transistor having base, emitter and collector electrodes, the collector electrode being coupled to the other of said differential amplifier output terminals, the base electrode being coupled to the base electrode of said first transistor; said emitter electrodes of said first and second transistors being coupled together to a source of potential; first switching means for connecting the base electrode of said first transistor to the collector electrode of said first transistor during a first time period, and to the collector electrode of said second transistor during a second time period; second switching means for connecting said signal output terminal to the collector electrode of said second transistor during said first time period; and to the collector electrode of said first transistor during said second time period; and, control means for cyclically controlling the switching of said first and second switching means.
 2. The electronic circuit of claim 1 further comprising:a third switching means for connecting said one output terminal of said differential amplifier to the collector electrode of said first transistor during said first time period, and to the collector electrode of said second transistor during said second time period; and said control means coupled to said third switching means for cyclically controlling the switching of said third switching means. 